Keep all Animations Nodes instances in one layer
The answer is pretty simple, create a loop and add a Object list for Iterator and connect your object list from the Object Transform Output or whatever list you are using to the iteration, and use the Add > Object > Layer Visibility Output node and connect your Object output to it. and use the Type of "Single" and the number of your layer in the Layer Index, and if you want to keep it on several layers use the Multiple Type and select your desired layers.
Remember that layers have the index equal to the layer number minus one, so, they go from 0 to 19.
Illustrative image: